This invention relates to seismic event protection systems, and more particularly, to systems for protecting clarifier feedwells containing water or wastewater from sudden hydrodynamic loading events resulting from seismic activity.
In those parts of the world where seismic activity is common, there is increasing concern about the effects of earthquakes on the infrastructure supporting communities, including water and wastewater treatment facilities. Earthquakes can cause structural damage to hydraulic systems at water or wastewater treatment facilities, by rupturing or otherwise damaging piping and/or equipment. These hydraulic systems, once damaged, can take months to repair or replace. In the meantime, the community affected by the earthquake may be forced to permit sewage to discharge to receiving waters untreated, posing significant burdens on the public health and the environment.
One example of a hydraulic system in a water or wastewater treatment facility is a clarifier. A clarifier is a quiescent basin or tank through which a water or wastewater is passed for removal of suspended solids (typically called xe2x80x9cprimaryxe2x80x9d clarifiers) and removal of biological flocs in activated sludge (typically called final, or xe2x80x9csecondaryxe2x80x9d clarifiers) by gravitational settling. The removal of settleable suspended solids by this method is known as clarification, or alternately, sedimentation. FIG. 1 shows a cutaway view of a circular secondary clarifier in accordance with the prior art. Influent wastewater to be treated enters the clarifier via an influent pipe 8 which runs underneath the clarifier 10, rises at the center, and discharges at the inlet well 12 at a high velocity. As the water spills over inlet well 12 to feedwell 14, its velocity is significantly reduced. Water then underflows feedwell 14 and flows radially outward toward the wall 15 of clarifier 10. Feedwell 14 has sludge rakes 16 attached to it, and slowly rotates as it operates, being driven by a drive unit (not shown). The sludge rakes 16 are mounted in pairs, with each pair forming a vee (V) in plan view. As sludge rakes 16 move, the sludge which accumulates at the bottom of clarifier 10 is collected in each vee and removed by uptake sludge pipes 17 mounted above sludge rakes 16. The sludge is discharged from sludge pipes 17 into a sludge box (not shown) in the center of clarifier 10. The sludge is discharged from the sludge box by pumping. The treated water spills over overflow weir 20 into clarified effluent channel 22. The clarified effluent is then further treated by other processes downstream.
Clarifiers may hold very large volumes of water or wastewater, with diameters of between 50 and 60 feet and heights between 8 and 12 feet being common. Consequently, if the volume of water in a clarifier is suddenly shifted, as may be caused by seismic activity, a large hydrodynamic load may result. Typically, seismic activity generates a wave within the clarifier. When the wave hits the feedwell within the clarifier (the feedwell typically being constructed of solid steel), it hits the feedwell with such force that the feedwell and the sludge rakes attached to the feedwell actually undergo plastic deformation, becoming a twisted mass of steel. The drive unit for the feedwell is also typically destroyed. As a result, the wastewater treatment facility cannot remove solids from incoming wastewater until the clarifier is repaired.
To get the clarifier back in service after an earthquake, the clarifier must be entirely drained, the feedwell and sludge rakes replaced and the drive unit repaired or replaced, as appropriate. This process may take several weeks or even months. In the meantime, the water or wastewater treatment facility has no ability to produce a sludge from the influent wastewater stream, and wastewater from the facility will be discharged to a receiving water body untreated.
Seismic protection systems of which Applicants are aware include two commercially available feedwells. In the first commercially available feedwell, the feedwell is constructed of stainless steel mesh, rather than solid steel. Even during normal operation conditions, this feedwell does not operate as well as a solid steel feedwell, because as the stainless steel mesh feedwell rotates, the stainless steel mesh vibrates, creating turbulence in the water inside the feedwell. This turbulence radiates outward toward the wall of the clarifier, which prevents the sludge from settling properly within the clarifier. While this feedwell will not undergo plastic deformation as a result of the hydrodynamic load due to a seismic event (since the wave of water hitting the mesh will simply pass through it), it does not function well as part of a sedimentation basin for water and wastewater treatment systems.
In another commercially available feedwell, the feedwell is constructed of a very brittle material, such as fiberglass. During normal operating conditions, this feedwell works as well as a solid steel feedwell from the standpoint of sedimentation. If this feedwell experiences a sudden, high magnitude hydrodynamic load due to a seismic event, rather than undergoing plastic deformation, the feedwell simply shatters in place, with the shattered material sinking to the bottom of the clarifier. However, to get a clarifier having this feedwell back in service following a seismic event, the clarifier must be drained down, the shattered material must be removed, and a replacement feedwell must be purchased and installed. This process may take weeks or months to complete, just as with a solid steel feedwell.
Clearly there is a need for a feedwell design which can withstand high magnitude hydrodynamic loads resulting from seismic activity, such that the clarifier may quickly be brought back into service, so as to protect the public health, and to minimize the effects of seismic activity on the operational capacity of a water or wastewater treatment system, and on the environment of the receiving water receiving the effluent from the treatment system.
A clarifier feedwell having a seismic protection system is described. The clarifier feedwell includes a feedwell frame and at least one pressure relief door which is slidably inserted into the feedwell frame. The pressure relief door includes a supporting frame, the supporting frame defining an opening. A flexible panel is disposed in the opening. A first edge of the panel is attached to a first portion of the supporting frame. The remaining edges of the panel are releasably engaged with other portions of the supporting frame such that the panel occupies substantially the entire area of the opening. At least one stiffening frame is disposed on the panel. The stiffening frame increases the rigidity of the panel, so that the panel does not disengage from portions of the supporting frame under a normal hydrodynamic load. However, under a sudden high magnitude hydrodynamic load, the flexible panel disengages from selected portions of the supporting frame, allowing water to pass through the clarifier feedwell without damaging it. In one embodiment, the pressure relief door and the panel are rectangular.